As it is known by one skilled in the art, certain electronic devices, such as mobile communication devices, and notably mobile phones, have one or more audio signal amplification paths, such as microphone and audio accessory amplification paths that are subjected to a burst noise, which disturbs the audio signals. This burst noise mainly comes from the radio signals that are transmitted by the device antenna through bursts at different frequencies, such as 900 MHz and 1800 MHz (square-wave signal with Tcycle=4.615 ms Cyclic ratio ⅛ modulated by radio signal at a frequency of 900 MHz, for instance). This modulated signal is picked up by the microphone and/or audio accessory, but also by the conductive lines connecting the microphone and/or audio accessory connector to a baseband processor.
This burst noise is usually demodulated by the baseband processor's inner amplifiers and/or the microphone, which pollutes the audio signals to be transmitted.
For rejecting the burst noise one generally uses single or differential capacitor components (with chosen capacitances). These capacitor components are generally connected differentially between the two lines of each microphone path and single-ended between each line and a ground line, just before the baseband processor inputs but also just after the microphone outputs.
These capacitor components comprise packages that act as parasitic inductances and resistors, so that the capacitor components act as RLC serial cells. Now, the resonant frequencies of these RLC serial cells are approximately equal to the radio signal frequencies (900 MHz and 1800 MHz), so that they act as short-circuits.
Moreover, since the tracks where the components are soldered bring additional resistive (R) and inductance (L) components to their RLC model, new capacitance values must be computed for each capacitor component each time the layout of the mobile phone Printed Circuit Board (PCB) is modified, which is time-consuming and, therefore, expensive for mobile phone development. Moreover, the computed capacitance values may differ from the ones that are effectively adapted to the device, so that a lot of empiric tests are generally needed to get the required performance. The work and number of PCB-trials needed for tuning the burst noise immunity of microphone paths with discrete solution are one of the most, and sometimes the most, critical steps in terms of planning during a mobile phone development.
Moreover, when the amplification paths are protected against the burst noise, they usually need additional specific protection for the electrostatic discharges (ESD) and interface cells for AC-coupling and biasing purposes that are ensured by extra components.
Finally, these capacitor components, in addition to components needed for interfacing the microphones and the baseband component, such as AC-coupling capacitor for DC level adaptation, biasing resistor for biasing the microphones and Electro Static Discharge (ESD) protections, can occupy a substantial area on the board. This area occupancy prevents efficient layout of the PCB.